How far down the rabbit hole do you want to go? At some point, you'll have to learn quantum mechanics and relativity, among other things. Otherwise, you'll just have to make assumptions and fudge a few equations to get the output you want. You could make mass and radius whatever you want, or you could base one or both of them from the mass and volume and binding habits of the atoms and molecules in your universe, under the influence of gravity. Luminosity comes from nuclear fusion mostly, which you could replicate or approximate as you wish!

I don't know much about physics, but it's fun playing. As a start, I think it would be best to get one physical phenomenon working before you try and make the whole universe. Gravity seems like the obvious choice. Atomic forces are more complicated. Nuclear fusion sounds like a doozy.